Gordon, perhaps they were allowed in because:
1) No one asked them what their attitudes were towards non-Muslims?
2) No one asked them if they intended to be Terrorists 10-20 years after the arrived?
3) No one asked them to swear allegiance to Australia when they arrived?
4) No one asked them not to listen to Islamist preachers who preached hatred towards all non-Muslims?
5) Perhaps they became radical in reaction to events overseas which occurred after they came to Australia, such as 11 September, the attack on Afghanistan/Iraq/etc.?
Gordon, no government is perfect, no government policy is perfect, there will always be mistakes. People change over time, their attitudes change because of circumstances and their reactions to them. A person who is a good migrant might decide to start supporting say, Hitler and create a Nazi cell of agents who are dedicated to sabotaging our nation's war effort against the Axis powers (and don't claim I'm being ridiculous - it happened - look up Johannes Heinrich Becker - father to the SA MP Heini Becker).
You OTOH believe that everybody should be gifted with perfect foresight and determine peoples' fate purely on the basis of Racial/Religious/Sexual prejudice. Fortunately most people think you're a fool. I know I do.